The present invention relates to a light-emitting element array device and a light-emitting element driver circuit employed as a light source for a recording operation in a printing apparatus of an electronic photography, and in particular, to a light-emitting element array device, a light-emitting element driver circuit, and a light-emitting element array driver integrated circuit (IC) suitable for controlling a driving current volume in a multi-stage fashion.
FIG. 10 shows an example of a driving operation of a light-emitting diode array as an example of a prior-art light-emitting element array. The configuration of FIG. 10 includes a driver IC 1 as a light-emitting element driver circuit, a light-emitting diode array LED, and a limiting resistor R. As can be seen from FIG. 10, a constant-current driving operation is achieved by inserting a limiting resistor in series. The conventional driver IC 1 employed in this constitution operates as follows.
In the driver IC 1 having an input terminal DI for a shift register SR and a shift clock terminal CLK, image data is sequentially fetched into the shift register SR in response to the shift clock. A latch circuit LATCH temporarily holds the image data fetched into the shift register SR and is operated in accordance with an input latch strobe from a terminal LST. DST indicates an input terminal to enable drivers D.sub.1 to D.sub.N, and a driver strobe signal therefrom is used to open a gate G so as to set the pertinent driver to a conductive state when the output data from the latch LATCH is "1". Q.sub.1 to Q.sub.N are output terminals of the respective drivers D.sub.1 to D.sub.N and function as power supply circuits to conduct electric power to the light-emitting diode array LED via the respective limiting registers Rs. The current conducted to each light-emitting diode is a constant current of ##EQU1## determined by the power source voltage E and a forward-directional voltage drop in the light-emitting diode V.sub.F. DO indicates a serial output terminal of the shift register SR, which is disposed to transfer image data to the next driver IC because the light-emitting diode array head usually includes several thousand of elements and hence many driver ICs are used.
In FIG. 10, reference numeral 10 designates a light-sensitive drum on which a latent image of printing information is produced by lights emitted from the light-emitting diodes in accordance with the image data.
A circuit of this kind has been described, for example, in the JP-A-61-185981.
A light-emitting diode array is formed on a single wafer according to the semiconductor technology. Ordinarily, since a light output power of the light-emitting diode array greatly varies in the overall wafer, a portion thereof in which the variation of the light emission power is small is cut out as a chip to be used. In a light-emitting diode array head in which a plurality of such separate semiconductor chips are arranged, the fluctuation of the light emission power is reduced; however, there still remain variations in the light emission among elements and among chips. Consequently, when an image of the electronic photography is produced with such a diode array head driven by a constant current, there appears a variation in the density thereof, which leads to a problem that the picture or image quality is deteriorated.
In addition, the light emission power of the light-emitting diode array may be attended with a change that becomes apparent with a lapse of time in some cases. As a result, the density of the image of the electronic photography is increased or decreased with the lapse of time. Consequently, it thereby becomes necessary to change the driving current with the lapse of time. Since the output current volume cannot be controlled in the conventional driver IC chip, a variation in the limiting resistance must be used to cope with this problem. The limiting resistor is manufactured on a print head substrate on which the chip is mounted and, as a result, it would be difficult to work on the limiting resistor to set the resistance value thereof again, which leads to a problem of the conventional technology that the limiting resistor cannot be corrected with respect to the change associated with the lapse of time.
In the conventional driver IC, in order to shift data in a bidirectional fashion, the flip-flop circuits FFs required for the shift register are twice as many as those employed for the single-directional operation. As a result, the size of the driver IC is necessarily increased and hence the size of the light-emitting array head on which the driver IC is mounted is also increased, which leads to the problem of the prior art that the cost of the array head is soared.